Omega Leonis is a real star in the constellationLeo, located 112 light years away from Earth. According to Star Trek: Star Charts (p.16), a sector block is a roughly cubical area of space 100 light years wide/long and 80 light years high. Each block contains 100 sectors.
The designation "block 27" was seen as part of the transporter destination coordinates on the portable transwarp beaming device display graphic. The alternate designation "Omega Leonis sector block" was seen in the targeted comm burst display graphic on the main viewscreen of the USS Enterprise.